1. Field of Invention
The present invention relates to springs with an improved corrosion-fatigue strength.
2. Description of Related Art
From the viewpoint of environmental protection and resource protection, it is strongly demanded to reduce the amount of toxic substances in exhaust gas and improve the fuel efficiency of automobiles. Since these are achieved by reducing the weight of an automobile to a considerable extent, efforts for reducing the weight of every automobile part are constantly underway. In the case of springs, including suspension springs, an increase in the employing stress (or the design stress) of a spring directly contributes to its weight reduction. But the increase in the employing stress usually causes fatigue and sagging of the spring. This problem is not limited to the springs used in automobiles, but also occurs to springs used in general machines.
Many kinds of spring material have been proposed so that that include various alloying elements to improve the fatigue resistance and sagging resistance. In the Publication of Unexamined Japanese Patent Application No. H3-2354, for example, a spring steel is disclosed including a lesser amount of Carbon, but including Nickel and Chromium and including a greater amount of Nitrogen than conventional ones. It is further proposed to include, in addition to those prescribed, one or more among the elements of Niobium, Vanadium and Molybdenum.